SWrapBox using parent's size as Preferred Size?

Development Programming & Scripting C++
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How to make SWrapBox use its parent’s size as the preferred size? So that if I change the window size, it automatically re-arrange its children.
I looked into the code and it seems like it’s being constructed with a default value of 100 if the parameter is not specified, and it does not seem to support what I want.

/*SWrapBox.h*/
SLATE_BEGIN_ARGS(SWrapBox)
		: _PreferredWidth(100.f) 
		, _PreferredSize(100.f)   //default value here
		, _InnerSlotPadding(FVector2D::ZeroVector)
		, _UseAllottedWidth(false)
		, _UseAllottedSize(false)
		, _Orientation(EOrientation::Orient_Horizontal)
		{
			_Visibility = EVisibility::SelfHitTestInvisible;
		}